The present invention relates to a communication system having at least two types of communication channels which has a central station and a plurality of terminal stations connected through a bi-directional communication path such that the terminals communicate with each other through the central station. More particularly, the present invention is directed to a communication system suitable for use in multimedia applications utilizing a wide variety of information forms such as audio information, output data of computers, text information, graphics, moving pictures, and so on.
In recent years, communication systems utilizing CATV (Community Antenna TV) networks have drawn attention as a new communication infrastructure in Japan as well as in the other countries. An example of a communication system responding to this trend is an MCA/C telephone system which is also given a common name of "analog CATV telephone" in a practical world. Since the MCA/C telephone system is an application of a multi-channel access (hereinafter abbreviated as "MCA") scheme based on the FM radio communication technology, which has already given positive achievement in the field of radio communications, to the CATV network, the name "MCA/C" was derived from MCA with "C" as the meaning of "on Cable" added thereto.
The MCA/C telephone system utilizes a 2.5 MHz band for each of uplink and downlink in a CATV transmission path to provide 200 communication channels at carrier frequencies located at intervals of 12.5 KHz, as shown in a main specification table of FIG. 11.
An example of the conventional MCA/C telephone system is illustrated in FIG. 7. Specifically, FIG. 7 is a block diagram illustrating an example of the MCA/C systems of a type which shares part of facilities for a CATV system for operation. The illustrated system comprises a head end 1 positioned in a central portion of the CATV system and a plurality of trunk line cables 2-1-2-n extending from the head end 1 to different directions.
The head end 1 is provided with an input/output distributor unit 101 from which the trunk line cables 2-1-2-n branch out. The input/output distributor unit 101 is connected to CATV broadcasting facilities, generally designated by 102, and a block convertor 103 through which a line control unit 104 and a communication management unit 105 are connected to the input/output distributor unit 101.
While the trunk line cables 2-1 - 2-n are extended in every direction, it is a general tendency to divide the trunk line cables in a plurality of directions in order to limit the number of subscribers accommodated therein and the length of cables. The trunk line cables in the respective directions are extended by inserting bi-directional relay amplifiers 3-1, 3-2 at multiple stages.
Branch cables branched from the bi-directional relay amplifiers 3-1, 3-2 have tap-off units 4-1, 4-2 from which the branch cables are further branched to respective subscribers 5-1, 5-2, thus configuring a tree type network as a whole.
Customer stations of the respective subscribers 5-1, 5-2 each include a distributor 501, a television receiver 502, a network terminal unit 503, a telephone 505, and a personal computer having a function of facsimile (hereinafter abbreviated as "FAX/PC") 506, such that the subscribers 5-1, 5-2 are permitted to receive television programs on multiple channels as well as to utilize the telephone, FAX/PC and so on for intra-communications within the system. The intra-communications employ a bi-directional transmission scheme in which a direction from subscribers to the head end is defined "uplink" and a direction from the head end to subscribers is defined "downlink".
Instead of the MCA/C telephone system, the development of a system generally called a "digital CATV telephone" has been advanced in recent years to follow the trend of digitization in the world. A time-division multiple access (hereinafter abbreviated as "TDMA") scheme is employed as a means for implementing the digital CATV telephone.
In a digital CATV telephone system under development, a communication channel has a bandwidth equal to the bandwidth assigned to one channel of the NTSC television standard.
Examples of the TDMA schemes for the digital CATV telephone systems are given below:
1 An exemplary system utilizes an entire 6 MHz bandwidth and performs quadrature phase shift keying modulation (hereinafter abbreviated as "QPSK modulation") to ensure a transmission rate of approximately 8 Mbps, so as to realize a TDMA scheme which provides a unit transmission rate of 64 Kbps for each terminal.
2 Another exemplary system divides a 6 MHz bandwidth into four 1.5 MHz subchannels and performs QPSK modulation in each of the subchannels to ensure a transmission rate of approximately 2 Mbps, so as to realize a TDMA scheme which provides a unit transmission rate of 64 Kbps.
FIG. 10 illustrates a conceptual block diagram for explaining a line connecting operation in TDMA telephone communications utilizing a CATV network. Specifically, uplink time slots (channels) ts1, ts2, . . . , tsn and downlink time slots (channels) Ts1, Ts2, . . . , Tsn are shown along a transmission path. A terminal station 51 uses an uplink time slot ts1 for transmission and a downlink time slot Ts1 for reception, respectively, while a terminal station 52 uses an uplink time slot ts2 for transmission and a downlink time slot Ts2 for reception, respectively. A switching between an uplink time slot and a downlink time slot, represented by broken line arrows in the drawing, is performed by a switching unit 41 and a multiplexer unit 42 in a head end 40. Also, the terminal stations 51, 52 have network terminal units 513, 523 connected to a CATV transmission path and telephones 515, 525 connected to the network terminal units 513, 523, respectively.
FIG. 12 is a table listing main specifications for the TDMA scheme illustrated in FIG. 10. The table shows that the TDMA scheme of FIG. 10 utilizes a 6 MHz bandwidth for each of uplink and downlink in the CATV transmission path, and forms four subchannels which are assigned four carrier frequencies located at intervals of 1.5 MHz to provide 24 channels per subchannel and a communication channel having a total of 96 channels.
The first MCA/C telephone system used in practice as a general-purpose communications system utilizing a CATV network has a limit in its utilization. On the other hand, currently developed digital CATV telephones are now in a transition phase of the development. The TDMA schemes employed in so far disclosed digital CATV telephone systems have the following features and problems in relation to the characteristics of CATV networks, utilization form, business form, and so on.
First, the characteristics of CATV networks will be described.
(1) Bi-directional CATV networks having a tree-type network suffer from noise ingress (noise which has intruded into a distribution system and a customer line system and flows into a head end) in an uplink transmission band which is a peculiar phenomenon in the CATV networks. Thus, a transmission scheme exhibiting a higher noise resistance is required for the uplink transmission band in comparison with the transmission of television video signals and so on in a downlink transmission band.
(2) While the noise ingress may occur due to interference of radio equipment or the like in a particular band, a main cause of the noise ingress is generally considered to be an intrusion of noise generated from home electric appliances, industrial machines, or the like. An analysis of such noise with a spectrum analyzer or the like shows that impulse noise having narrow width and a short duration randomly occurs on the frequency axis. It can be said from this observation that a transmission scheme using a wider bandwidth is more likely to suffer from interference of the noise.
(3) In conventional CATV networks, even if bi-directional facilities are provided, the uplink transmission band including up to extreme ends of cables has been utilized less frequently than the downlink transmission band. Therefore, no attention has been paid to countermeasures to noise ingress in existing CATV networks, and a variety of experiments have gradually revealed that the level of noise ingress is generally in an unfavorable situation.
To improve the level of noise ingress, it is necessary to examine all terminals one by one to confirm sources of noise intruding into the uplink, to reinforce the shielding for cables themselves and contact portions of coaxial cables, and so on. However, since a single CATV system includes several thousand to fifty thousand terminals to be checked and repaired, a huge cost is anticipated to take countermeasures to the noise.
(4) The CATV network must also cope with noise possibly generated in its own network in addition to noise intruding from the outside as mentioned above. Such noise generated in the network is mainly caused by non-linearity of transmission paths.
Generally, CATV networks perform relay amplification at multiple stages in order to compensate for line loss. Thus, the CATV network has a problem of inter-channel interference generated by cross modulation, intermodulation, and so on due to non-linearity of the relay amplifiers, depending on the characteristics of relay amplifiers and the number of relay stages. As examples of countermeasures to prevent the influence of inter-channel interference from exceeding a tolerable limit, total transmission power is limited per channel of the television, and the number of channels is limited when a bandwidth assigned to one channel of the television is used in a frequency division multiple access scheme.
Since trunk line cables have been gradually replaced with optical fibers in CATV networks, a less number of relay stages is required so that the non-linearity of transmission paths tends to be improved as an overall network.
(5) It has been found that when wide-band high-speed digital transmission is performed in a CATV network, reflected waves caused by impedance miss-match of transmission paths may exert influences on the CATV network. To solve this problem, it would be necessary to modify the entire network even including customer lines and terminal equipment at extreme ends, so that an ideal modification of the network is anticipated to be difficult from the viewpoint of a cost required therefor.
(6) In CATV networks, a single coaxial cable is used to communicate uplink and downlink signals therethrough. Generally, CATV networks in Japan employ an uplink band from 10 MHz to 50 MHz and a downlink band from 70 MHz to 450 MHz (or 750 MHz). Even if the entire uplink band is available for transmission, a total number of available channels, each assigned a 6 MHz bandwidth, is merely six. In addition, the above-mentioned noise ingress is not present uniformly over the entire band but concentrates in a band from 10 MHz to 25 MHz. Thus, channels in such a noisy band cannot be used in practice. In the United States, the uplink band is narrower than that in Japan and is assigned a range of 10-30 MHz.
It can be understood from the circumstance mentioned above that the uplink channels are precious and should be efficiently utilized.
Next, the characteristics of the MCA/C telephone system will be described.
(7) Since the MCA/C telephone system employs a narrowband transmission scheme as compared with the characteristics of transmission paths used in the CATV network as mentioned above, it can advantageously provide fairly good transmission and a larger number of channels. However, the MCA/C telephone system has a disadvantage that the data transmission rate is limited to approximately 9,600 bps and cannot provide services which require higher transmission rates than this limit.
Next, the TDMA scheme has the following problems.
(8) As mentioned above, since digital CATV telephone systems employing currently known TDMA schemes use a wide band such as 6 MHz or 1.5 MHz, they are susceptible to the influence of noise ingress having the nature as mentioned above. In addition, the digital CATV telephone systems are disadvantageously susceptible to the influence of reflections due to impedance miss-match because of the wide-band high-speed transmission.
(9) Noise levels at respective terminal stations distributively located in a tree-type CATV network are not uniform but exhibit large variations depending on directions of trunk lines to which they are connected. However, to ensure a uniform transmission quality in an entire system, it is desirable to maintain a ratio of a carrier level to a noise level (C/N) at the same level at all terminal stations. A scheme which employs separate carriers to respective communication channels can employ weighted power distribution which assigns a higher transmission level to a communication with a terminal station connected to a path with a higher noise level within a tolerable range of total transmission power for one television channel. However, it is difficult to apply the weighted power distribution to the TDMA scheme which transmits a plurality of communication channels with a single carrier.
(10) In the TDMA scheme, PCM transmission at a rate of 64 Kbps is performed on a channel among analog telephones connected to an associated network. As a result, the TDMA scheme is inferior to the MCA/C telephone system in terms of a frequency utilization efficiency per 6 MHz. Specifically, when comparing these schemes in terms of the number of channels constituting an audio transmission band (0.3-3.4 KHz), the TDMA scheme has 96 channels while the MCA/C telephone system has 200 channels. Even in this case, the MCA/C telephone system utilizes only a limited 2.5 MHz band within the 6 MHz band.
(11) The CATV telephone systems typically employ a fixed rate system in order to avoid a huge facility cost which would otherwise be required to a meter rate telephone accounting management system. The fixed rate system charges the same telephone rate for a predetermined period irrespective of an actual utilization amount. There is a demand for a CATV telephone system which can accommodate as many subscribers as possible at a lower rate. The telephone communications schemes may be classified into the following two. A first scheme is an instantaneous communications scheme, such as a line connection scheme for the existing subscriber telephone system, in which once two terminals are connected through a line, the terminals occupy the line so that any other terminal is not permitted to access to the line until a call between the two terminals is terminated. A second scheme is a delay base scheme, as employed by LAN (local area network), in which a common line is assigned to a plurality of terminals such that each terminal is given a waiting time to access to the common line. In personal computer communications, the data transmission rate is determined by a signal transmission rate of each terminal even if a maximum signal transmission rate of a communication line is high. Also, even when computer terminals are connected through a communication line, there may be a period in which no data is actually transmitted therebetween. Thus, a line utilization ratio for some data communications between personal computers may generally indicate a small value. The latter communications scheme such as that employed by LAN may be appropriate for subscribers who have a data processing apparatus such as a computer connected to a CATV telephone system.
To increase the line utilization ratio, it is advantageous to employ a scheme which accommodates many terminals in a single channel and provides a delay base communications for these terminals, as is actually implemented in LAN. In such a LAN-type communications scheme, efficient utilization of lines can be accomplished by absorbing a difference in rate between terminals and lines and by allowing other terminals to utilize a line during a non-operating time during which the line is occupied but not actually utilized. However, in the TDMA scheme which relies on an instantaneous line switching scheme, communications with an increased line utilization ratio is difficult to realize.
(12) In addition, the TDMA scheme requires the assignment of two time slots, i.e., two channels for respective uplink and downlink in order to provide full-duplex communications, similarly to the assignment of audio channels in an MCA/C scheme, later described. In the LAN-type communication scheme, which assumes the employment of half-duplex communications, a bi-directional communication line can be constituted of only one channel.
While a variety of forms can be thought for data communications, most of them are data transmission from one terminal to another terminal. A system which reserves an empty line for always allowing for bi-directional simultaneous communications is disadvantageous in terms of the line utilization efficiency.
(13) The TDMA scheme employed in the CATV telephone system is based on 64 Kbps per channel. Generally, network terminal units at extreme ends of the network do not have a rate converting function, so that a terminal unit operating at a transmission rate different from that of the line requires an adaptor for the rate conversion. Since it is assumed in LAN that the rate of lines is basically different from that of terminals, the terminals in LAN are adapted to flexibly correspond to any transmission rate.
Finally, problems implied in LAN will be described.
(14) For example, in a LAN system as indicated by the LAN standard IEEE 802.3, a terminal connected to a node is allowed to transmit at any moment under predetermined conditions, and there is no means provided for controlling a total amount of traffic in a network such as regulation of transmission or the like.
When a LAN type communications scheme is applied to a CATV network, it is necessary to assume a system capable of accommodating a great number of subscribers. For example, it is thought that the CATV network should have a function of limiting new communication requests (regulating the assignment of these requests to a busy channel) in a traffic condition in which a waiting time exceeds a predetermined value.
(15) Some data supplied through a CATV network may often require fast and continuous data transmission, as is the case of a moving image. A communications scheme like LAN, which is designed basically for intermittent data transmission in packet units, is not suited to fast and continuous data transmission. To solve this problem, it will be necessary to occupy a channel in a condition close to line switching to enable the provision of continuous data transmission service depending on the form of information transmission.
Such a service is not intended for all of ordinary subscribers. For example, in an at-home welfare service, this service may be limited for particular utilization, for example, communications between hospitals or welfare centers and solitary elders' houses.
(16) The LAN system generally employs a transmission scheme which uses an entire band of a cable serving as a common transmission path. There has not been found any example which divides an entire band to form a plurality of channels for providing different services on the respective channels.